1. Field of the Invention
This invention relates, in general, to the field of magnetoresistive spin-valve (SV) devices. More particularly, the present invention relates to a current perpendicular-to-the-plane (CPP) spin-valve device configuration, applicable as a magnetic non-volatile mass-memory called xe2x80x9cMRAM,xe2x80x9d and/or as a magnetic transducer or xe2x80x9cheadxe2x80x9d for reading information signals recorded on a magnetic medium.
2. Description of the Related Arts
The related art discloses a magnetic transducer referred to as a spin-valve (SV) magnetoresistive sensor, e.g., in U.S. Pat. No. 5,159,513, issued on Oct. 27, 1992. An SV element includes at least two ferromagnetic layers with different magnetic field sensitivity, the one of the higher sensitivity being called a xe2x80x9cfree layerxe2x80x9d and the other a xe2x80x9cpinned layer.xe2x80x9d The resistance depends on the angle between the magnetizations of the two layers as cosine of the angle and is independent of the current direction with respect to the magnetization directions, which is traditionally called the giant magnetoresistive effect (GMR). In MRAM""s, the information of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d is stored either by the direction of the magnetization in the free layer or in the pinned layer. The stored information is read out by applying a field and sensing the sign of the change of the resistance of the SV element. See, for instance, xe2x80x9cCharacteristics of AP Bias in Spin Valve Memory Elements,xe2x80x9d J. Zhu and Y. Zheng, IEEE Trans. Magn. Vol. 34, 1063-1065 (1998); xe2x80x9cPseudo Spin Valve MRAM Cells with Sub-Micrometer Critical Dimension,xe2x80x9d B. A. Everitt et al, IEEE Trans. Magn. Vol. 34, 1060-1062 (1998). An SV element can also be applied to a magnetic field sensor, detecting a magnetic field through the resistance changes of the element. It has been shown that an SV head is capable of reading data from a magnetic surface at great linear densities.
The GMR effect is larger when measured with a current perpendicular to the layer structure than when measured with a current in plane direction. See, for example, xe2x80x9cPerpendicular Giant Magnetoresistance of Microstructured Fe/Cr Magnetic multilayers from 4.2 to 300 K,xe2x80x9d M. A, M. Gijs, S. K. J. Lenczovski and J. B. Giesbers, Physical Review Letters, Vol. 70, 3343-3346 (1993). The former is called CPP (current perpendicular-to-the-plane) and the latter, CIP (current-in-plane). Therefore, it is desirable to make use of CPP rather than CIP if the circumstances allow. However, the drawback in the CPP structure is the fact that only a very low resistance change is obtainable compared to the CIP structure making it necessary to apply a very large current to obtain a reasonable output signal level.
Commercial value arises from an improved CPP (current perpendicular-to-the-plane) spin-valve element that enhances the performance of MRAM""s and magnetic read heads, especially in its high output characteristics with a low current.
An objective of the present invention is to provide a spin-valve (SV) device of a CPP structure which has a high resistance, and which generates a high output signal with a low current.
The invention further advantageously provides an easy method for enabling the CPP spin-valves to be put into practical use, by having a potential of almost double the signal to noise ratio of the CIP (current-in-plane) spin-valve of other devices.
In accordance with the present invention, an SV device having a layered structure called an xe2x80x9cSV elementxe2x80x9d formed on a substrate includes first and second thin film layers of ferromagnetic materials separated by a thin layer structure of non-magnetic material. In the present invention the magnetization of one of the layers of a ferromagnetic material, called a xe2x80x9cfree layer,xe2x80x9d is easier to change its direction than that of the other of the layers of a ferromagnetic material, called a xe2x80x9cpinned layer,xe2x80x9d by application of a magnetic field and means to produce a current flow between the free and pinned layers through a conducting part of the non-magnetic layer structure. The area of the conducting part of the non-magnetic layer structure being smaller than the area of the free layer, thus making the resistance between the free and pinned layers much higher than otherwise. The resistance of the SV element changes as a function of the angle between the magnetizations in the free and pinned layers. For memory applications, the information of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d is stored by the direction of the magnetization either in the free layer or in the pinned layer. The stored information is read out by applying a field and sensing the sign of the change of the resistance of the SV element. When applied to a field sensor such as a read head in magnetic recording devices, the resistance change of the SV element is sensed due to the difference in rotation of the magnetizations in the free and pinned layers as a function of the magnetic field being sensed. The present invention provides a way of obtaining substantially higher output by applying a lower current than that obtained by devices in the related art.